System and method for controlling electric fields in electro-hydrodynamic applications

ABSTRACT

An electro-hydrodynamic system that extracts energy from a gas stream, which includes an injector that injects a first species of particles having the same polarity into the gas stream, wherein particle movement with the gas stream is opposed by a first electric field; an electric field generator that generates a second electric field opposing the first, such that the net electric field at a predetermined distance downstream from the injector is approximately zero; an upstream collector that collects a second species of particles having a polarity opposite the first particle species; a downstream collector that collects the charged particle; and a load coupled between the downstream collector and the upstream collector, wherein the load converts the kinetic energy of the gas stream into electric power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 13/276,055, filed 18 Oct. 2011, which claims the benefit ofU.S. Provisional Application No. 61/394,298, filed 18 Oct. 2010, andU.S. Provisional Application No. 61/528,440 filed 29 Aug. 2011, whichare incorporated in their entirety by this reference.

This application is related to prior application Ser. No. 12/357,862,filed 22 Jan. 2009, and prior PCT application number PCT/US09/31682,filed 22 Jan. 2009, which are incorporated in their entirety by thisreference.

TECHNICAL FIELD

This invention relates generally to the electro-hydrodynamic wind energyconversion field, and more specifically to a new and useful system andmethod in the electro-hydrodynamic wind energy conversion field.

BACKGROUND

Electro-hydrodynamic (“EHD”) wind energy conversion is a process whereinelectrical energy is extracted directly from wind energy. An EHD systemis typically a solid-state device that uses wind energy to act againstan electrostatic field, separating charged elements from a chargedsource. In concept, this system can convert wind kinetic energy toelectrical potential energy in the form of charges collected at veryhigh voltages.

Past investigations into this field, however, have been fraught withmany problems that rendered the energy collection insufficient whencompared to the energy input for operating the EHD system. Inparticular, as an EHD system collects charge from the separation of thecharged particles, the system creates an electric field (also called asystem field 120) that opposes the motion of the charges. The systemfield may cancel and even overwhelm the electric field used to chargethe particles in the EHD system. As a result, the charge supplied to acharged element (e.g., droplets in a charged liquid spray) is reduceddue to the interference of the system field with the charging field.This lowers the working current and power output of the entire system.Additionally, the charged particles that are emitted to the wind streamencounter a very large opposing electrostatic force (also called a spacecharge 122, shown in FIG. 1), created by the cloud of previouslyreleased charged particles downwind from the injector exit, whichpromotes shorting of the droplets to the charging elements or othercomponents rather than entrainment in the wind stream where the chargedparticle can contribute to energy harvesting as shown in FIG. 2. Thus,there is a need in the electro-hydrodynamic wind energy conversion fieldto create new and useful systems and methods for controlling magnitudeand direction of the electric field in electro-hydrodynamicapplications. This invention provides such a new and useful system andmethod.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of a prior art electro-hydrodynamicenergy extraction system with a space charge.

FIG. 2 is graph of prior art indicating unsatisfactory electrical fieldcharacteristics.

FIGS. 3A and 3B are a schematic representation of a preferred embodimentof the invention and a schematic representation of the electrical fieldsof a preferred embodiment of the invention, respectively.

FIG. 4 is an exemplary graph of electrical field characteristics of apreferred embodiment of the invention.

FIG. 5 is a schematic representation of an injector of a preferredembodiment of the invention.

FIG. 6 is a schematic representation of a first embodiment of theinvention.

FIG. 7 is a schematic representation of a second embodiment of theinvention.

FIG. 8 is a schematic representation of an embodiment of the inventionincluding an aerodynamic electric field generator.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments of the inventionis not intended to limit the invention to these preferred embodiments,but rather to enable any person skilled in the art to make and use thisinvention.

1. System for Field Shaping in EHD Applications

As shown in FIG. 3, a system 100 for controlling electric fields (i.e.,field shaping) in an electro-hydrodynamic (EHD) application of apreferred embodiment includes an injector 200 and an electric fieldgenerator 300. The injector 200 (or “charge generator”) preferablyincludes at least one electrode 220 and a nozzle 240, and morepreferably an array of electrodes 220 and nozzles 240 as shown in FIG.6. As shown in FIG. 3B, the electric field generator 300 generates anelectric field (shaping field 320) that opposes the system field 120(i.e., has the same field direction as the charging field 201). Theshaping field preferably reverses the electric field near the injector,and causes the net electric field to fall to substantially zero at apredetermined point downstream from the injector (minimum field point).The system functions to diffuse the space charge 122, resulting from acloud of charged particles localized about the injector 200, byreversing the electric field in the immediate vicinity of the injector200 (e.g., spray nozzle). As shown in FIG. 4, by maintaining anelectrostatic field near the injector 200 with the same field directionas the charging field, the field shaping system has the effect ofreducing interference with the charging process as well as controllingand optimizing the charge induced on the particles. Furthermore, bymaintaining a minimum field point downstream from the injector 200, theshaping field of the electric field generator 300 minimizes the shortinglosses by attracting the charged particles away from the injector over ashort distance and promoting entrainment in a gas stream 10 (e.g., windstream). For example, when generating positively charged particles, theelectric field generator will preferably maintain a positive field atthe tip of a nozzle 240 even when the system voltage drops below ground.The system will preferably maintain field magnitude and direction (i.e.,field shape) during normal operation conditions of an EHD system. Thefield reversal of the electric field generator is preferably highlylocalized (preferably about the injector), such that the wind may stilldo work against the system field 120 downwind from the field reversal.Furthermore, the direction of the field reversal changes to oppose thedroplet motion at a predetermined distance downstream from the spraysource to support energy extraction under the prevailing conditions.Depending on the design of the electric field generator 300 and injector200 (e.g., nozzle type and particle type), the field reversal induced bythe electric field generator may have other shape characteristics. Thesystem ultimately functions to increase the amount of energy that can beharvested from an EHD system. While the preferred system is for windenergy harvesting with an EHD system, the system may alternatively beadapted for other suitable EHD applications such as charge suppressionin the presence of multiple charged sprays in the field of agriculturalspraying or industrial painting. As shown in FIG. 3, the systempreferably additionally includes an upstream collector 140, a downstreamcollector 160, and a load 180. The upstream collector 140, positionednear the injector 200, collects a particle 204 of opposite charge(polarity) to the particle released into the gas stream. The downstreamcollector 160, positioned downstream from the injector, collects theparticle released into the gas stream. The load 180, electricallycoupled between the upstream and downstream collectors, facilitatescharged particle flow between the two collectors (i.e. current flow),and transforms the current flow into electric power. The upstreamcollector is preferably a portion of the injector, but may alternativelybe any suitable collector. The downstream collector is preferably aconductive mesh, but may alternatively be the ground (e.g. earth), orany suitable collector. The load is preferably a resistive load, morepreferably an adjustable resistive load, but may be any suitable load.The parameters of the load and upstream and downstream collectors(resistance, separation distance, angular orientation, orientationrelative to the wind, voltage, etc.) are preferably adjustable andcontrolled by a processor (e.g. a CPU, a computer control system, etc,)to promote energy extraction from the prevailing conditions.

The injector 200 of the preferred embodiment functions to impart chargeon a particle and to introduce the charged particle 202 into theelectric field. The charged particle is preferably suitable forentrainment within a fluid stream, more preferably a wind stream, suchthat energy may be harvested from the work done on the particle by themoving fluid. The particle is preferably a water droplet but may be anysuitable particle as described below. The injector preferably injectscharged particles into the field. The injector is preferably arrangedsubstantially parallel to the gas stream, but may alternatively beoriented perpendicular to the gas stream (e.g. vertically, horizontally,etc.) at an angle to the gas stream, or in any suitable orientation. Theinjector 200 is preferably an electrospray injector, but may be ahydrostatic injector, a dry needle (e.g. that injects a substantiallydry, charged particle into the wind), a corona charger, or any suitableinjector. The injector 200 preferably includes an electrode 220 and anozzle 240 as shown in FIG. 5, wherein the electrode 220 charges theparticles emitted from the nozzle 240. More preferably, the injector 200includes a plurality of electrodes 220 and nozzles 240. The injectors inone embodiment are preferably substantially similar to the injectorsdescribed in prior application Ser. No. 12/357,862, filed 22 Jan. 2009,titled “ELECTRO-HYDRODYNAMIC WIND ENERGY SYSTEM” and prior PCTapplication number PCT/US09/31682, filed 22 Jan. 2009, titled“ELECTRO-HYDRODYNAMIC WIND ENERGY SYSTEM” which are incorporated intheir entirety by this reference. The injector 200 may alternatively beany suitable device and/or process that locally induces charge on aparticle or spray.

The electrodes 220 of the preferred embodiment function to maintain ahigh field concentration at a particle entrance (e.g., a nozzle tip ornozzle exit) to charge the particles. The particles are preferably waterdroplets that are inductively positively charged. The electrode 220 ispreferably a rail electrode or a ring electrode (i.e., induction ring)made of conductive material. A ring electrode 220 preferablycircumscribes the nozzle, and is preferably aligned with the ring axisconcentric with the axis of the nozzle 240, with the planar positionvariable before, co-planar, or aft of the tip of the nozzle 240. Thering electrode 220 preferably maintains a higher field concentration ata point closer to the nozzle than to the electrode, wherein the highfield concentration preferably has a rapid drop-off of field(volts/meter) heading radially outward towards the ring electrode fromthe central axis of the ring electrode. This preferably facilitates highfield strengths for charging of particles (e.g., electrospray) withoutproviding a current path for short-circuiting. Use of a large diameter(e.g., ⅛-¼ inch) cross-section wire or rod to form the ring electrode220 preferably improves the shape of the electric field lines such thatcharging occurs, but short circuiting is reduced. The electrode 220 mayadditionally be aerodynamically shaped. For example, the cross sectionmay have an airfoil shape. A large diameter ring electrode additionallypreferably provides sufficient space between a nozzle and the electrode220, creating a large area for wind to carry away charged particles.However, the electrode 220 may alternatively be any other suitableelectrode (e.g. a plate electrode) and have any other suitable formfactor.

The nozzle 240 of the preferred embodiment functions to produceparticles to be charged and entrained within the wind stream. The nozzlepreferably emits liquid droplets. The droplets are preferably waterdroplets but may alternatively be water plus additives (e.g. surfactant,cesium, etc.), a water solution, or any suitable alternative liquid.Alternatively, the nozzle may emit a substantially dry particle, such asa polymer or fertilizer. The nozzle preferably has a nozzle tip on thedistal end of the nozzle 240 through which the particle is emitted. Apump system is preferably attached to the nozzle on the proximal end ofthe nozzle 240. A wide variety of nozzle types may be used due to thereduction in restrictions in spray/droplet requirements due to the fieldreversal from the electric field generator 300. The nozzle is preferablyone of those disclosed in PCT application number PCT/US09/31682, but mayalternatively be any suitable nozzle for particle emission and charging.As an alternative to the nozzle 240, air, particulate matter, or othernon-liquid particles may alternatively be introduced for charging.

Additionally or alternatively, a preferred embodiment may have aplurality of injectors 200 within the field of the electric fieldgenerator 300. The injectors 200 are preferably arranged in an array,but may alternatively be arranged in one or more rows, columns, rings,or any other suitable configuration. The field of the electric fieldgenerator 300 is preferably useable by any suitable number of injectors.The array of injectors 200 are preferably aligned along a plane,preferably within the space within the electric field generator 300, butalternatively in a space upstream or downstream from the electric fieldgenerator 300. The number of injectors is preferably dependent on theproperties of the electric field generator 300. In one example, 8-12injectors are co-planarly arranged within the field of the electricfield generator.

The electric field generator 300 (field shaper) of the preferredembodiment functions to manage the properties of the electric field(magnitude and direction) in the region substantially near the injector.More specifically, the electric field generator 300 functions togenerate a shaping field 320 that reverses the system field 120 in alocalized space near the injector. The net electric field at the tip ofa nozzle 240 (or at another satisfactory portion of the injector 200) ispreferably held at the maximum electric potential of the charging field(e.g. significantly positive or negative). The magnitude of the netelectric field preferably precipitously drops from this maximum at thetip of the nozzle 240 to zero at a point downstream from the injector(zero field point 322). This zero field point is preferably 5 to 10millimeters displaced from the tip of the nozzle 240 in the direction ofthe wind stream. The zero field point may alternatively be displaced agreater length, such as 20-50 millimeters or any suitable distance.Thus, the net electric field preferably transitions from a chargingfield at the nozzle tip to pulling the particles at a point downstreamfrom the injector 200. This functions to prevent shorting of theparticles. Beyond the zero field point, the net electric field thenpreferably opposes particle motion along the Z-axis (i.e., in thedirection of the wind stream). Energy can then be harvested from thewind stream overcoming the opposing electric field force on the chargedparticle. The net electric field at the zero field point 322 ispreferably approximately zero, but may alternatively be slightlynegative or slightly positive. The electric field generator 300 (alsoknown as a field shield or field shaper) is preferably a conductiveelement (e.g. a guard electrode) charged to create an electrostaticfield with constant shape during normal operating conditions. However,the electric field generator 300 may be an electromagnetic generator,such as a magnetic element (e.g. permanent magnet or electromagnet), orany suitable electric field generator. Additionally/alternatively, thegenerated electric fields may be dynamic and time-variable instead ofstatic. The electric field generator may be positioned substantiallyco-planar, downwind, or upwind relative to the injector 200.

In a first embodiment, as shown in FIG. 6, the electric field generator300 is preferably a circumscribing structure 340 with an open spacedefined within the center of the electric field generator. The injector200 is preferably located substantially within this defined open space.The electric field generator is preferably an electrode (e.g. guardelectrode), and is preferably made of conductive material, and may bemade to be substantially similar to that of the electrode 220, onlylarger in proportion. The electric field generator is preferably alignedwith the axis of the open space to be substantially co-centric andco-planar with the injector 200 (e.g., the electrode 220 and tip of thenozzle 240) as shown in FIG. 6. The electric field generator 340 ispreferably a large inductive ring adjacent to the plane of the tips ofthe nozzle 240. The electric field generator is preferably toroidal inshape, but may be any suitable shape, such as intersecting tubes withrounded ends. The cross sectional diameter of the tubes or ring ispreferably substantially larger than those of the electrodes 220. Thespace defined within the electric field generator 340 is preferablylarge enough for an injector 200 and may additionally be large enoughfor a plurality of injectors 200 arranged in an array as describedbelow. In one variation, the electric field generator 300 forms astructural component of the system. For example, the electric fieldgenerator 300 may additionally function as a frame coupling thecomponents of the system. As another example, the electric fieldgenerator may be used as a conduit or enclosure for fluid lines,electrical input/return, connectors, or any suitable portions of thesystem EHD system. The lines are preferably channeled through a cavityon the inside of the electric field generator and are properlyinsulated.

The system of the first embodiment may additionally include at least onefield leveler 342 that functions to homogenize (i.e., normalize or makeuniform) the field of the electric field generator. The field leveler342 is preferably a conductive component that augments the field of theelectric field generator 340. The field leveler 342 is preferably usedin combination with an array of injectors 200. For example, a fieldleveler 342 is preferably positioned within the center of the spacedefined by the electric field generator 340 as shown in FIG. 6. Theinjectors 200 towards the center of the array that are farthest from theelectric field generator 340 may be less protected than those adjacentto the electric field generator 340. The field leveler 342 is preferablya conductive element with the same voltage as the electric fieldgenerator that “levels out” the field such that injectors towards thecenter are more equally protected. A plurality of field levelers 342 maybe used. The field levelers 342 are preferably charged to a voltagesignificantly lower than the electrodes 220. This preferably enables alarger coverage area than the electric field generator 340 may becapable of providing on its own.

In a second embodiment, as shown in FIG. 7, the electric field generator360 includes an attracting electrode 364 and a shielding electrode 362of opposing polarities. A shaping field 320 is created between theattracting-shielding electrode pair, wherein the shaping field 320interacts with the charging and system fields to create a zero fieldpoint 322 substantially near the electric field generator, downwind fromthe injector 200. The attracting electrode 364 and shielding electrode362 are preferably closely positioned (preferably coupled by adielectric couple but alternatively positioned by any other means). Theattracting electrode 364 has a polarity opposite that of the chargedparticles and functions to the charged particles away from the injector200. The shielding electrode 362 has polarity similar to the chargedparticles and functions to prevent particle shorting to the attractingelectrode by repelling the charged particles away from the attractingelectrode. To attract the particles away from the injector, theattracting electrode preferably has a larger electric potentialmagnitude than the shielding electrode. For example, if positiveparticles are released, the attracting electrode is preferably morenegative than the shielding electrode is positive such that its effectdominates the shaping field 320. The electric field generator 360 ispreferably positioned downwind from and near the injector, with theshielding electrode proximal to the injector and the attractingelectrode distal from the injector. The electrode pair 360 is preferablyaligned substantially against the system field, but may be aligned alongthe wind stream. However, the electric field generator 360 may have anyother position (e.g., fore or co-planar relative to the injector 200),any other orientation (e.g. the shielding electrode may be distal andthe attracting electrode proximal to the injector 200), or any otherrelative electric potential magnitudes (e.g. the shielding electrode maybe more positive than the attracting electrode is negative). In onespecific embodiment, as shown in FIG. 7, the positively chargedparticles are attracted to the relatively large negative attractingelectrode in the far field (B), but are repelled from shorting to theattracting electrode 364 when the particles enter the near field (A) bythe positive shielding electrode 362. Particle momentum (imparted by thegas stream and attraction to the attracting electrode) preferablyprevents the particle from shorting to the attracting electrode as theparticle flows pas the attracting electrode. The shielding andattracting electrodes are preferably bar electrodes, but mayalternatively be point electrodes, a combination thereof, or anysuitable type of electrode. Additionally, parameters of the electrodepair (e.g. separation distance between the electrodes, position relativeto the injector, electric potentials of the electrodes, etc.) may bedynamically altered to support energy extraction under the prevailingconditions. The system may include any number of electric fieldgenerators 360 arranged in any configuration (row, column, array, ring,etc.). In a specific embodiment, the system includes a row of electricfield generators 360, aligned in parallel with the injectors within thesystem field.

Additionally, the electric field generator 300 may be adapted to alterone or more properties of the fluid stream. More preferably, theelectric field generator 300 is adapted to have aerodynamic and/orlift-generating properties that focus the wind stream to enhance chargedparticle radial expansion to further dissipate the space charge. Windfrom a wider area, preferably upstream from the injector 200, may befunneled through the electric field generator 300. The electric fieldgenerator is preferably formed as an airfoil, but may alternatively beformed as a convergent-divergent nozzle (as shown in FIG. 8), whereinthe injector 200 is located in the throat of the nozzle, a convergentnozzle, a divergent nozzle, or have any other suitable aerodynamic form.These aerodynamic properties may additionally be inherent in the ringshape of the electric field generator 300. The depth of the electricfield generator 300 may additionally be set to facilitate the extensionof the protected field area. An additional device may be coupled to theelectric field generator 300 to further widen the area of windcollection. Any suitable device or technique may alternatively be usedto facilitate control of aerodynamic properties in the vicinity of theinjector(s) 200. The electric field generator 300 may additionallyand/or alternatively alter the temperature, humidity, pressure, or anyother suitable parameter of the gas stream flowing therethrough.

The electric field generator 300 may be made of modular components suchthat an electric field generator can be easily constructed. A modulardesign could even be designed for connecting an array of electric fieldgenerators. The electric field generator 300 preferably is rounded orhas rounded edges. Sharp edged hardware such as nuts and fasteners arepreferably protected, and common corona reduction practices arepreferably followed. The electric field generator may alternatively comein any suitable shape or form. The electric field generator ispreferably electrically insulated to prevent shorting, and is preferablyencapsulated by a solid dielectric material, but may alternatively beencapsulated by a liquid dielectric material, glass, ceramic, or acomposite polymer.

The electric field generator 300 is preferably powered to generate theelectrostatic field of the electric field generator 300. With theaddition of the electric field generator, the electrodes 220 of theinjector 200 can preferably be operated at a lower voltage, whichfunctions to increase efficiency. Lowering the voltage even furtherbelow that of the electrode(s) 220 may further lower the inductioncharging voltage and enhance the efficiency of the system. Additionally,the electric field generator 300 may be charged by a power sourceseparate from the ring electrode(s) 220. With a separate power supply,the electric field generator 300 preferably draws power during startupand preferably maintains the field with no or little power loss sincethere is preferably no current flowing in the circuit of the electricfield generator 300. This is comparable to charging a capacitor andholding the voltage constant. Small amounts of positive entrained chargemay short back to the electric field generator 300. The aerodynamicdesign of the electric field generator may reduce such occurrences.

In a first preferred embodiment, as shown in FIG. 6, the system includesan injector, an upstream collector, a downstream collector, a loadcoupled between the upstream collector and the downstream collector, andan electric field generator. The injector is an array of electrosprayinjectors, each including a ring electrode circumscribing a nozzle thatemits a plurality of positively charged particles into the gas stream.The injector also functions as an upstream collector that collects thenegatively charged particles orphaned by the released particles. Thedownstream collector collects the positive particles at a pointdownstream from the injector. A first electric field (system field) thatopposes particle movement with the gas stream is created in the spacebetween the upstream collector and the downstream collector, wherein thegas stream does work on the particle against the system field as thestream moves the particle from the injector to the downstream collector(e.g. drag on the particle by the gas stream at least partially opposesdrag on the particle by the electric field). The load, coupled betweenthe upstream collector and the downstream collector, functions toconvert the kinetic energy of the gas stream into power by facilitatingcharge flow (i.e. current flow) between the two collectors. The electricfield generator is an inductive electrode (e.g. ring electrode)circumscribing and substantially co-planar with the injectors. Theelectric field generator creates a second electric field (shaping field)opposing the first, such that the net electric field is a fallingpositive field, adjacent to the injector, which draws the positiveparticles away from the injector. At a zero field plane downstream fromthe injector, the net electric field transitions to a rising positivefield that opposes particle movement with the gas stream. The electricfield generator may additionally include a field leveler, positionedconcentric and coplanar with the electric field generator (preferablysubstantially in the middle of the injectors), that is held at the samepolarity as the electric field generator, that adjusts the net electricfield to be substantially homogeneous (in magnitude and direction)across the injector.

In a second preferred embodiment, the system includes substantially thesame components as the first embodiment, except that the electric fieldgenerator includes a plurality of shielding and attracting electrodepairs 360, located downstream from the injector, in addition to thecircumscribing inductive electrode. The electrode pairs are encapsulatedwithin an airfoil made of dielectric material. Each shielding electrodeis held at substantially the same low positive potential, and eachattracting electrode at substantially the same high negative potential(relative to the shielding electrodes). The magnitudes of the electricfields created between the shielding electrode and the attractingelectrode (shaping field) are preferably substantially smaller than themagnitude of the system field. The shielding and attracting electrodepairs 360 are positioned within the system field (downstream from theinjector), substantially near the injector, with the shielding electrodealigned proximal the injector and the attracting electrode distal theinjector. Each shielding and attracting electrode pair is substantiallyaligned along the gas stream, and moves as the gas stream changesdirection.

In a third preferred embodiment, the system includes substantially thesame components as the second embodiment, except that the electric fieldgenerator only includes the plurality of shielding and attractingelectrode pairs 360, and does not include a circumscribing inductiveelectrode.

As a person skilled in the art will recognize from the previous detaileddescription and from the figures and claims, modifications and changescan be made to the preferred embodiments of the invention withoutdeparting from the scope of this invention defined in the followingclaims.

We claim:
 1. An electro-hydrodynamic system that extracts energy from agas stream, the system including: an injector that injects a firstspecies of particles having the same polarity into the gas stream,wherein particle movement with the gas stream is opposed by a firstelectric field; an electric field generator that generates a secondelectric field opposing the first, such that the net electric field at apredetermined distance downstream from the injector is approximatelyzero; an upstream collector that collects a second species of particleshaving a polarity opposite the first particle species; a downstreamcollector that collects the charged particle; and a load coupled betweenthe downstream collector and the upstream collector, wherein the loadconverts the kinetic energy of the gas stream into electric power. 2.The system of claim 1, wherein the first species of particles areinjected from an injector exit, wherein a portion of the electric fieldgenerator is positioned upstream from the injector exit.
 3. The systemof claim 2, wherein the electric field generator at least partiallyencircles the injector.
 4. An electro-hydrodynamic system that extractsenergy from a gas stream, the system including: an injector that injectsa first species of particles having the same polarity into the gasstream, wherein particle movement with the gas stream is opposed by afirst electric field; an electric field generator that generates asecond electric field opposing the first, wherein the second electricfield reverses the net electric field a predetermined distancedownstream from the injector, the second electric field having a higherstrength at a point distal from the electric field generator than apoint proximal to the electric field generator; an upstream collectorthat collects a second species of particles having a polarity oppositethe first particle species; a downstream collector that collects thecharged particle; and a load coupled between the downstream collectorand the upstream collector, wherein the load converts the kinetic energyof the gas stream into electric power.
 5. The system of claim 4, whereinthe electric field generator is an electrode.
 6. The system of claim 5,wherein a portion of the electric field generator is positioned upstreamfrom the injector.